BEHAVIOR AND CAPACITY OF THERMALLY RESTRAINED MOMENT FRAME MEMBERS DURING COMPARTMENT FIRE

Abstract

In the event of a fire hazard, structural resistance depends on the ability to resist a comprehensive set of load combinations comprising the gravity loads and the thermally induced fire loads without undergoing failure. Structural floor beams and connections of moment frames are typically designed against flexural demands from gravitational or lateral loads, and axial loads induced due to restrain against thermal expansion (thermal restraint) are generally not part of the design considerations. Prior studies on the fire behavior of structural steel frames have indicated that the thermally induced axial loads on beams and connections are dependent on the temperature change and connection type. Due to the high stiffness of moment (rigid) connections, these thermally-induced axial loads, in addition to beam bending moments due to gravity loads, can lead to various failure modes such as local buckling, plastic hinging, or other instabilities in the member or connection. Although there have been research studies on the flexural response of moment connection at elevated temperatures, the combined effect of axial force-bending moment interaction at elevated temperature under a thermal restraint condition has not been thoroughly investigated. This thesis focuses on investigating the structural behavior of floor beams and moment connections that are part of moment frames under the combined effect of bending moment and thermally-induced axial force during a fire event. Finite element analysis method was employed to investigate the structural behavior of members and connections under elevated temperatures as a result of compartment fire. Benchmark finite element models were first developed to verify the modeling approach using data from past experimental research. The numerical models accounted for temperature-dependent material models by Eurocode and NIST developed models, and also incorporated damage and failure criteria in the constitutive material models. Member level studies of typical floor beams (slender sections for compression) under combined bending and axial loading were conducted to investigate strength through finite element analysis. The analysis results were used to develop interaction capacity curves for combined axial and bending moment cases at elevated temperature (M-N-T) and compared against the member strength equations provided in the AISC specification Appendix 4 for elevated temperature design of steel structures. It was observed that the AISC equations overestimate the strength of slender members for compression at low slenderness ratios. The analysis results also demonstrated that the beam-column design equations including the combined effects of axial-load and bending moment provided reasonable strength estimate for slenderness ratio of 60 and above. Connection capacity studies were conducted on a typical welded unreinforced flange-bolted web (WUF-B) connection as a representative moment connection. The moment connection behavior was primarily governed by the failure modes exhibited at the ends of the connecting floor beams, therefore the interaction curves developed for beam-column member strength using the AISC provisions resulted in providing conservative estimates and are recommended for usage in moment connections capacity calculations during fire conditions.